An organic light-emitting diode (OLED), also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes. An OLED device includes a substrate, an anode, an organic hole-transporting layer, and an organic luminescent (emission) layer with suitable organic dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. This multilayer OLED device is discussed in commonly-assigned U.S. Pat. Nos. 4,769,292 and 4,885,211.
Organic materials, thickness of vapor-deposited organic layers, and layer configurations, useful in constructing an OLED device, are described, for example, in commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292, the disclosures of which are herein incorporated by reference.
Organic materials useful in making OLED devices, for example organic hole-transporting materials, organic light-emitting materials predoped with an organic dopant, and organic electron-transporting materials can have relatively complex molecular structures with relatively weak molecular bonding forces, so that care must be taken to avoid decomposition of the organic material(s) during physical vapor deposition.
The aforementioned organic materials are synthesized to a relatively high degree of purity, and are provided in the form of powders, flakes, or granules. Such powders, flakes or granules have been used heretofore for placement into a physical vapor deposition source wherein heat is applied for forming a vapor by sublimation or vaporization of the organic material, the vapor condensing on a substrate spaced apart from the deposition source to provide an organic layer thereon.
Several problems have been observed in using organic powders, flakes, or granules in physical vapor deposition:                (i) powders, flakes, or granules are difficult to handle because they can acquire electrostatic charges via a process referred to as triboelectric charging;        (ii) powders, flakes, or granules of organic materials generally have a relatively low physical density (expressed in terms of weight per unit volume) in a range from about 0.05 to about 0.2 g/cm3, compared to a physical density of an idealized solid organic material of about 1 g/cm3;        (iii) powders, flakes, or granules of organic materials have an undesirably low thermal conductivity, particularly when placed in a physical vapor deposition source which is disposed in a chamber evacuated to a reduced pressure as low as 10−6 Torr. Consequently, powder particles, flakes, or granules are heated only by radiative heating from a heated source, and by conductive heating of particles or flakes directly in contact with heated surfaces of the source. Powder particles, flakes, or granules which are not in contact with heated surfaces of the source are not effectively heated by conductive heating due to a relatively low particle-to-particle contact area; and        (iv) powders, flakes, or granules can have a relatively high ratio of surface area/volume of such particles and a correspondingly high propensity to entrap air and/or moisture between particles under ambient conditions. Consequently, a charge of organic powders, flakes, or granules loaded into a physical vapor deposition source which is disposed in a chamber must be thoroughly outgased by preheating the source once the chamber has been evacuated to a reduced pressure. If outgasing is omitted or is incomplete, particles can be ejected from the source together with a vapor stream during physical vapor-deposition of an organic layer on a structure.        
Other types of imaging devices, such as imaging phosphors for computed radiography and x-ray photoconductive devices for digital radiography, depend on the ability to coat the active materials uniformly over large areas. While the following discussion pertains to OLED displays, it should be readily apparent that the same invention can be applied to the deposition of alkalihalide phosphors, amorphous semiconductors, and other luminescent or photoactive layers, as well as a variety of other materials used in devices based on such luminescence or photoactive layers.
For sufficiently small substrates, a point source approach can be implemented wherein the material to be deposited emanates from a localized heated crucible and the substrate is placed far enough from the localized region of vaporization so that the coating is sufficiently uniform along the substrate. As substrate size increases or working distance increases, rotary or planetary motion of the substrate relative to the localized source is often required to produce the desired uniformity.
By elongating the vaporization source and providing for translation of source and substrate relative to one another, the desired uniformity can be attained at considerably smaller working distances and thus considerably higher rates and better materials utilization, if desired. Scaling of such a process to large areas (i.e. substrates greater than 15 cm in at least one dimension) is considerably easier than for point sources.
An elongated source (also known as a linear source) for thermal physical vapor deposition of organic layers onto a structure for making an organic light-emitting device has been disclosed by Spahn in commonly assigned U.S. Pat. No. 6,237,529. The source disclosed by Spahn includes a housing, which defines an enclosure for receiving solid organic material, generally in powder form, which can be vaporized. The housing is further defined by a top plate which defines a vapor efflux slit-aperture for permitting organic vapors to pass through the slit onto a surface of a structure spaced apart from the elongated source. The housing defining the enclosure is connected to the top plate. The source disclosed by Spahn further includes a conductive baffle member attached to the top plate. This baffle member provides line-of-sight covering of the slit in the top plate so that organic vapors can pass around the baffle member and through the slit onto the substrate or structure while particles of organic materials are prevented from passing through the slit by the baffle member when an electrical potential is applied to the housing to cause heat to be applied to the solid organic material in the enclosure causing the solid organic material to vaporize.
In using the thermal physical vapor deposition source disclosed by Spahn to form an organic layer of a selected organic material on a substrate or structure, it has been found that the vapor efflux slit-aperture causes non-uniform vapor flux of organic material to emanate along a length dimension of the slit. There is a problem when the width dimension of the slit is reduced, for example, to a width dimension less than 0.5 mm. Such spatially non-uniform orientation of opposing slit edges can be thought of as a deviation of planarity of opposing edges which, in turn, can promote a greater fraction of organic vapors to exit the vapor deposition source through a central portion of the slit, with a correspondingly lower fraction of organic vapors exiting the source through remaining portions of the slit along its length dimension. Such non-uniform vapor flux, directed at a substrate or structure, will cause the formation of an organic layer thereon which will have a non-uniform layer thickness in correspondence with the non-uniform vapor flux.
In addition, any nonuniformities in heat generation from the heater or heat absorption by the material to be deposited or distribution of the material within the source can give rise to nonuniformity in deposition along the length of the source. Yet another source of nonuniformity is unintended leaks in the source enclosure other than the apertures used to deliver the organic vapor. If such leak exists at the ends of the source., the flow of vapor from center to end of the source can cause pressure gradients within the source, thereby causing nonuniformity in the resultant deposition.
Forrest et al. (U.S. Pat. No. 6,337,102B1) disclosed a method of vaporizing organic materials and organic precursors and delivering them to a reactor vessel wherein the substrate is situated and delivery of the vapors generated from solids or liquids is accomplished by use of carrier gases. In one embodiment of their invention, Forrest et al. located the substrates within a suitably large reactor vessel, and the vapors carried thereto mix and react or condense on the substrate. Another embodiment of their invention is directed towards applications involving coating of large area substrates and putting several such deposition processes in serial fashion with one another. For this embodiment, Forrest et al. disclosed the use of a gas curtain fed by a gas manifold (defined as “hollow tubes having a line of holes”) in order to form a continuous line of depositing material perpendicular to the direction of substrate travel.
The approach to vapor delivery as disclosed by Forrest et al. can be characterized as “remote vaporization” wherein a material is converted to vapor in an thermal physical deposition source external to the deposition zone and more likely external to the deposition chamber. Organic vapors alone or in combination with carrier gases are conveyed into the deposition chamber and ultimately to the substrate surface. Great care must be taken using this approach to avoid unwanted condensation in the delivery lines by use of appropriate heating methods. This problem becomes even more critical when contemplating the use of inorganic materials that vaporize to the desired extent at substantially higher temperatures. Furthermore, the delivery of the organic vapor for coating large areas uniformly requires the use of gas manifolds.
A method of making compacted pellets using a mixture of organic materials comprising at least one host and one dopant to form an organic layer on a substrate which will form an OLED display is disclosed by Ghosh et al. in commonly assigned U.S. Pat. No. 6,649,436. The method disclosed by Ghosh et al. utilizes a point or a linear evaporation source wherein the mixture of organic materials is vaporized by heating from the top of the source. The advantages achieved by this method is that separate evaporation sources for the purpose of co-evaporation are not required for making an organic layer in making an OLED display. This method of deposition is strongly dependent on the vapor pressure and sublimation temperature of each individual organic component in the mixture. It is possible that compacted pellets using a mixture of two organic materials wherein one organic component has substantially a lower sublimation temperature as compared to the other organic component, will be prone to depletion of that organic component having lower sublimation temperature in a relatively short time. Markedly different sublimation temperatures of the organic components may render the compacted pellets unusable in a manufacturing environment because the evaporation sources are used continuously for hours in making OLED displays.
Each one, or a combination, of the aforementioned aspects of organic powders, flakes, or granules can lead to nonuniform heating of such organic materials in physical vapor deposition sources with attendant spatially nonuniform sublimation or vaporization of organic material and can, therefore, result in potentially nonuniform vapor-deposited organic layers formed on a structure.